1. Field of the Invention
The present invention relates to an intake apparatus incorporated in an internal combustion engine such as an engine installed in a vehicle or the like.
2. Description of the Background Art
An internal combustion engine such as an engine installed in a vehicle or the like is generally provided with an intake apparatus for supplying an air-fuel mixture to a combustion chamber. The intake apparatus has an intake passage that constitutes a communicating section between a surge tank and an intake port of a cylinder head. The intake passage is provided for each intake port (cylinder).
There are conventionally two types of intake apparatuses: one having a passage fixed to a prescribed length, and the other having a passage changeable in length (see Japanese Patent National Publication No. 2001-509562, and Japanese Patent Laying-Open No. 2001-082155).
During an intake stroke of the internal combustion engine, when an intake valve is opened to introduce an air-fuel mixture, the air within the intake passage is normally supplied into the combustion chamber with pulsation. With such pulsation of the air within the intake passage, the air flow contains a portion of high density and a portion of low density that appear alternately. The period of the pulsation of the air is determined according to selection of shape, length, cross section and the like of the intake passage. Meanwhile, the timing of opening/closing the intake valve is determined according to the engine speed.
If it is possible to arrange the high-density portion of the air at the latter part of the intake stroke, the intake air will continue to flow into the combustion chamber by virtue of inertia even after the piston moves past the bottom dead center, which is called the “inertial supercharging effect”. This improves intake volumetric efficiency of the combustion chamber, and hence, the output of the internal combustion engine.
In the intake apparatus of the variable-length type, the passage length can be changed in accordance with the operation states of the internal combustion engine such as the load, engine speed and the like, so as to ensure the above-described inertial supercharging effect over the entire operation region of the internal combustion engine. This can improve the intake volumetric efficiency of the combustion chamber.
The conventional techniques however have the following problems. Firstly, in the conventional example shown in Japanese Patent National Publication No. 2001-509562, although the passage has a variable length, its cross section is uniform from the upstream side to the downstream side in the air introducing direction. Thus, it would be difficult to improve the intake volumetric efficiency over the entire region from the low engine speed region to the high engine speed region of the internal combustion engine.
Specifically, if the cross section of the intake passage is set small in accordance with the amount of the air required at the low engine speed region of the internal combustion engine, it will be difficult to secure the amount of the air required at the high engine speed region of the internal combustion engine. On the contrary, if the cross section of the intake passage is set large in accordance with the air amount required at the high engine speed region of the internal combustion engine, it will be necessary to practically secure a maximum passage length to be adapted to the low engine speed region of the internal combustion engine. This would inevitably increase the size of the component(s) constituting the intake passage, which cannot be installed in a vehicle unless the corresponding space can be secured therein.
In the conventional example shown in Japanese Patent Laying-Open No. 2001-082155, as shown in FIGS. 1-3 thereof, the stationary pipe 9 and the in-out pipe 8 are different in inner diameter size from each other. This causes an unlevel portion in inner diameter at the inner end of in-out pipe 8, possibly degrading intake efficiency.